Tighter legal regulations on the content of pollutants in engine exhaust gasses have prompted engine builders to intensify their developmental efforts. Two developmental approaches are of particular interest, namely one of collecting the pollutants in a filter, and another of continuously breaking down the pollutants in a catalytic converter.
In accordance with the first approach, particulate filters have been installed on a regular basis for some time in passenger vehicles equipped with diesel engines in order to capture soot particles emitted by the diesel engine. Soot particles settle into capillaries of the filter, thereby causing the pressure drop at the filter to increase during operation; in the extreme case, the filter can become clogged. To prevent such clogging, the filter must be regenerated from time to time by burning off the soot particles deposited therein.
The filter must be heated to a great extent in order to burn off the soot. Basically, this can be accomplished by operating the engine under high load or via a suitable manipulation of the point in time when fuel is injected into the engine. For example, extremely retarded injection or injecting fuel into a cylinder of the engine after it has passed top dead center and the previously injected fuel has already been ignited are ways to generate exhaust gas that is very hot and/or contains unburned fuel and heats the filter directly or via catalytic combustion that occurs downstream of the engine.
Once the soot deposited in the filter starts to burn, the combustion thereof provides a more or less considerable portion of the heat required to sustain combustion. It is therefore important to regulate the supply of energy to the filter from the outside during filter regeneration to ensure that the temperature in the filter does not reach a point at which the filter material begins to melt or sinter. The smaller the dimensions of a particulate filter are, the more rapidly heat from the filter dissipates into the surroundings. Heat loss is therefore relatively low in the case of a large-size filter of the type used, for instance, on a high performance engine of an agricultural machine such as a tractor, a combine harvester, a forage harvester, or the like. Although regeneration can therefore be initiated using a relatively small quantity of fuel, there is a great risk that the heat released by the combustion of the stored soot will cause the filter to overheat and become damaged.
To reliably prevent such damage from occurring, the conditions under which regeneration takes place must be strictly controlled, and disturbing influences that can cause the filter temperature to deviate upward or downward, in the manner of load fluctuations of the engine, must be avoided. To enable such strict control to be performed, it is common to require that filter regeneration be carried out only when the engine is idling.
Due to this limitation, the engine or a machine driven by the engine cannot be used while regeneration is underway1. Regeneration that takes place at an inopportune time is bothersome to the machine operator since work must be interrupted. If a plurality of machines works together to perform a task, such as harvesting and hauling machines used to harvest a field, an interruption in the operation of one machine affects the other machines and can result in enormous economic losses.
In pursuing the second approach mentioned above, a proposal was made to reduce the quantity of pollutants, nitrogen oxides in particular, that are produced in combustion by injecting an ammonifier into the exhaust-system branch, which reacts with the nitrogen oxides contained in the exhaust gas. Since this technique is much more difficult to implement than filtering, it has not found widespread use in the mass market. However, assumptions are that it will become absolutely essential for large engines in order to meet the increasingly stricter standards for exhaust gas. Unfortunately the ammonifiers known today, which are in the form of aqueous urea solutions, do not remain stable over the long term. An operator is therefore unable to stockpile them in order to replenish the urea supply to an engine at any time as needed. Additionally, there is no sales infrastructure in place that would allow last-minute demands to be satisfied around the clock, seven days a week, as is the case for engine fuels. If the ammonifier suddenly runs out, it may therefore become necessary to interrupt operation of the engine at an inopportune time, which is precisely what happens when a filter becomes full.